Had the same issue and found an answer somewhere on Reddit, apparently once your character hits 35 years of age marriage is absolutely unthinkable and therefore the option simply dissappears.
If this is a bug or a feature I have no idea, would be nice to get some clarity on that part.
EDIT: If you can be bothered modding the dev console in you can use that to change your character's age back to 34 or something, then you'll be able to get all flirty again.